Method of producing a sputtering target

ABSTRACT

A sputtering target contains a target material including as constituent elements Ag, In, Te and Sb with the respective atomic percents (atom. %) of α, β, γ and δ thereof being in the relationship of 0.5≦α&lt;8, 5≦γ≦23, 17≦γ≦38, 32≦δ≦73, α≦γ, and α+β+γ+δ=100, and a method of producing the above sputtering target is provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 09/956,637filed Sep. 19, 2001, (now U.S. Pat. No. 6,652,806), which is a divisionof application Ser. No. 08/979,293 filed Nov. 26, 1997 (now U.S. Pat.No. 6,319,368), which is a division of application Ser. No. 08/622,681filed Mar. 26, 1996 (now U.S. Pat. No. 5,736,657).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a sputtering target for fabricating arecording layer of an optical recording medium; a method of producingthe sputtering target; a phase-change optical recording mediumcomprising a recording material in a recording layer thereof, whichrecording material is capable of causing changes in the phase thereof bythe application of a light beam thereto, thereby recording, reproducingand overwriting information therein; a method of forming the recordinglayer for the above-mentioned phase-change optical recording medium byusing the above-mentioned sputtering target; and an optical recordingmethod using the above-mentioned phase-change optical recording mediumwhich is suitable for the application of optical memory devices, inparticular, rewritable compact disk (CD-rewritable) application.

2. Discussion of Background

There is conventionally known a phase-change optical informationrecording medium which utilizes phase changes between a crystallinephase and an amorphous phase or between one crystalline phase andanother crystalline phase as one of the optical recording media whichare capable of recording, reproducing and erasing information by theapplication thereto of electromagnetic waves, such as a laser beam. Thiskind of phase-change optical information recording medium enables theoverwriting of information by the application of a single laser beamthereto, although such overwriting is difficult to conduct by theapplication of a single laser beam in magneto-optical memory using amagneto-optical recording medium. An optical system of a drive unit forthe phase-change type optical information recording medium can bedesigned simpler than that for a magneto-optical recording medium, sothat recently research and development of this kind of recording mediumhas been actively conducted.

As disclosed in U.S. Pat. No. 3,530,441, the so-called chalcogen-basedalloys, such as Ge—Te, Ge—Te—Sn, Ge—Te—S, Ge—Se—S, Ge—Se—Sb, Ge—As—Se,In—Te, Se—Te and Se—As, are conventionally used as recording materialsfor the phase-change optical recording medium. In addition, it isproposed to add an element of Au to the above-mentioned Ge—Te basedalloy to improve the stability and to increase the rate ofcrystallization of the recording material as disclosed in JapaneseLaid-Open Patent Application 61-219692. Furthermore, the addition of Snand Au to the Ge—Te based alloy, and the addition of Pd to the same arerespectively proposed in Japanese Laid-Open Patent Applications61-270190 and 62-19490 for the same purposes as mentioned above.Furthermore, recording materials comprising a mixture of Ge, Te, Se andSb, and a mixture of Ge, Te and Sb are respectively disclosed inJapanese Laid-Open Patent Applications 62-73438 and 63-228433, each ofwhich has specific composition ratios of constituent elements of therecording material for the improvement of the recording and erasingrepetition properties of the obtained recording medium.

However, none of the above-mentioned conventional phase-change opticalrecording media satisfies all the requirements for the phase-changerewritable optical recording medium.

Japanese Laid-Open Patent Application 63-251290 discloses an opticalinformation recording medium which comprises a recording layercomprising a compound of a multi-component system composed ofsubstantially three or more components in a single crystalline phase.The single crystalline phase of the compound of a multi-component systemcomposed of substantially three or more components is considered tocontain a compound with a stoichiometric composition, for example,In₃SbTe₂, in the recording layer in an amount of 90 atom % or more. Itis mentioned that recording and erasing characteristics can be improvedto some extent by the provision of this kind of recording layer.However, this optical information recording medium has the shortcomingsthat the erasability is low and the laser power required for recordingand erasing cannot be sufficiently reduced.

Furthermore, Japanese Laid-Open Patent Application 1-277338 discloses anoptical recording medium which comprises a recording layer comprising analloy with a composition represented by the formula of(Sb_(a)Te_(1-a))_(1-b)M_(b), wherein 0.4≦a<0.7, b≦0.2, and M is oneelement selected from the group consisting of Ag, Al, As, Au, Bi, Cu,Ga, Ge, In, Pb, Pt, Se, Si, Sn and Zn. The basic system of theaforementioned alloy is Sb₂Te₃, and the addition of a large excess of Sbto this composition in terms of atomic percentage enables high-speederasing operation and improves the repetition properties. The additionof the element M can further enhance the high-speed erasing performance.In addition to the above advantages, this reference asserts that theerasability by the application of DC light is increased. However, thisreference does not show any specific erasability obtained at theoverwriting operation, and according to the experiments conducted by theinventors of the present invention, the erasability in the course of theoverwriting operation is unsatisfactory and the recording sensitivity isinsufficient for use in practice.

Japanese Laid-Open Patent Application 60-177446 discloses an opticalrecording medium which comprises a recording layer comprising an alloywith a composition represented by the formula of(In_(1-x)Sb_(x))_(1-y)M_(y), wherein 0.55≦x≦0.80, 0≦y≦0.20, and M is oneelement selected from the group consisting of Au, Ag, Cu, Pd, Pt, Al,Si, Ge, Ga, Sn, Te, Se and Bi. In addition, Japanese Laid-Open PatentApplication 63-228433 discloses a recording layer of an opticalrecording medium, which comprises an alloy with a composition ofGeTe—Sb₂Te₃—Sb (excess). The sensitivity and erasability required forthe recording medium cannot be satisfied in any of the above-mentionedconventional optical recording media.

In addition, a recording layer of an optical recording medium is made ofa Te—Ge—Sb alloy, with the addition thereto of nitrogen atom, asdisclosed in Japanese Laid-Open Patent Application 4-163839; a recordinglayer is made of a Te—Ge—Se alloy, with the addition thereto of anitride comprising at least one element of Te, Ge or Se, as disclosed inJapanese Laid-Open Patent Application 4-52188; and a recording layer ismade of a Te—Ge—Se alloy, with nitrogen atom being adsorbed thereby.

However, there are still some problems remaining unsolved in thoseconventional optical recording media.

As previously mentioned, most important points to be improved in theconventional phase-change optical recording media are to enhance therecording sensitivity and erasing sensitivity, to prevent the decreaseof the erasability in the course of the overwriting operation, which iscaused by the portions remaining unerased, and to increase the life of arecorded portion and that of a non-recorded portion.

With the rapid spread of a compact disk (CD), a write-once compact disk(CD-R) capable of writing data therein only once has been developed andput on the market. However, if the user does not succeed in writing thedata in the CD-R, the CD-R becomes unavailable because it is impossibleto correct the data. To compensate such a shortcoming of the CD-R, arewritable compact disk is eagerly expected to be put to practical use.

A rewritable compact disk has been researched and developed by utilizingthe magneto-optical recording medium, but the compact disk thus obtainedhas the shortcomings that there is difficulty in satisfactorily carryingout the overwriting operation, and the compatibility with the CD-ROM orCD-R is poor. Since it is considered that a phase-change opticalrecording medium can ensure the compatibility with the CD-ROM or CD-Rmore favorably in principle when compared with the above-mentionedrewritable compact disk of a magneto-optical type, the application ofthe phase-change optical recording medium to the rewritable compact diskhas been actively researched and developed recent years.

Such a rewritable compact disk obtained from the phase-change opticalrecording medium is reported in some references, for instance,“Proceedings of the 4th Symposium on Phase-Change Recording” p. 70(1992), Furuya et al.; “Proceedings of the 4th Symposium on Phase-ChangeRecording” P.76 (1992), Jinno et al.; “Proceedings of the 4th Symposiumon Phase-Change Recording” p. 82 (1992), Kawanishi et al.; Jpn. J. Appl.Phys. 32 (1993) p. 5226, T. Handa et al.; “Proceedings of the 5thSymposium on Phase-Change Recording” p. 9 (1993), Yoneda et al.; and“Proceedings of the 5th Symposium on Phase-Change Recording” p. 5(1993), Tominaga et al. However, none of the rewritable compact disksreported in the aforementioned references is satisfactory with respectto the overall performance, such as the compatibility with CD-R,recording and erasing performance, recording sensitivity, maximum numberof repeated overwriting operations, maximum number of repeatedreproducing operations, and shelf stability. Those drawbacks are mainlyascribed to low erasability due to the composition and configuration ofa recording material employed in each compact disk.

Under such present conditions, there is increasing a demand fordevelopment of a phase-change recording material with high erasability,and high recording and erasing sensitivities, and in addition, aphase-change rewritable compact disk capable of exhibiting excellentoverall performance.

To meet the aforementioned demand, the inventors of the presentinvention have proposed Ag—In—Sb—Te based recording materials, forexample, as disclosed in Japanese Laid-Open Patent Applications 4-78031and 4-123551; Jpn. J. Appl. Phys. 31 (1992) 461, H. Iwasaki et al.;“Proceedings of the 3rd Symposium on Phase-Change Recording” p. 102(1991), Ide et al.; and Jpn. J. Appl. Phys. 32 (1993) 5241, H. Iwasakiet al.

It became apparent that the phase-change optical disk thus obtainedexhibited excellent performance. However, still more endeavors should bemade to produce a phase-change optical recording disk which is capableof securely ensuring the compatibility with the CD-R, and perfectlysatisfying the previously mentioned overall performance required for therewritable compact disk, and to establish the method of producing aphase-change optical disk capable of forming a new market.

SUMMARY OF THE INVENTION

Accordingly, a first object of the present invention is to provide asputtering target for fabricating a recording layer of an opticalrecording medium with improved C/N ratio, erasability, recordingsensitivity, and repetition properties.

A second object of the present invention is to provide a method ofproducing the above-mentioned sputtering target.

A third object of the present invention is to provide an opticalrecording medium with improved C/N ratio, erasability, recordingsensitivity, and repetition properties.

A fourth object of the present invention is to provide an opticalrecording medium which is capable of recording information and erasingthe same at a disk rotation linear speed of as low as 1.2 to 5.6 m/s,and applicable to a rewritable compact disk.

A fifth object of the present invention is to provide a method offorming a recording layer for the above-mentioned optical recordingmedium.

A sixth object of the present invention is to provide an opticalrecording method using the above-mentioned optical recording medium.

The first object of the present invention can be achieved by asputtering target comprising a target material comprising as constituentelements Ag, In, Te and Sb with the respective atomic percents (atom. %)of α, β, γ and δ thereof being in the relationship of 0.5≦α<8, 5≦β≦23,17≦γ≦38, 32 ≦δ≦73, α≦β, and α+β+γ+δ=100.

It is preferable that the target material for use in the above-mentionedsputtering target comprise Sb, and AgInTe₂ with a stoichiometriccomposition and/or a nearly stoichiometric composition having achalcopyrite structure and/or zincblende structure, and it is preferablethat the AgInTe₂ in the target material be in the form of crystalliteswith a particle size of 450 Å or less.

The second object of the present invention can be achieved by a methodof producing a sputtering target which comprises a target materialcomprising as constituent elements Ag, In, Te and Sb with the respectiveatomic percents (atom. %) of α, β, γ and δ thereof being in therelationship of 0.5≦α<8, 5≦β≦23, 17 ≦γ≦38, 32 ≦δ≦73, α≦β, andα+β+γ+δ=100, comprising the steps of fusing a mixture of Ag, In and Teelements at a temperature in the range of 550° C. to 850° C. to preparea fused mixture, rapidly cooling the fused mixture to prepare a solidlump, pulverizing the solid lump to prepare finely-divided particles,mixing the finely-divided particles with Sb, and sintering the mixtureof the finely-divided particles and Sb.

Alternatively, the second object of the present invention can also beachieved by a method of producing the sputtering target, comprising thesteps of fusing a mixture of Ag, In, Te and Sb elements at a temperaturein the range of 550° C. to 850° C. to prepare a fused mixture, andrapidly cooling the fused mixture to prepare a solid lump, pulverizingthe solid lump to prepare finely-divided particles, and sintering thefinely-divided particles.

In either case, it is preferable that the method of producing thesputtering target further comprise the step of carrying out the heattreatment at a temperature not higher than the melting point of themixture prior to the sintering step.

The third and fourth objects of the present invention can be achieved byan optical recording medium comprising a recording layer which comprisesa phase-change recording material, capable of recording and erasinginformation by utilizing changes in the phase of the phase-changerecording material in the recording layer, the phase-change recordingmaterial comprising as constituent elements Ag, In, Te and Sb with therespective atomic percents of α, β, γ and δ thereof being in therelationship of 1≦α<6, 7≦β≦20, 20≦γ≦35, 35≦δ≦70, and α+β+γ+δ=100.

In the above-mentioned optical recording medium, it is preferable thatthe phase-change recording material comprise AgSbTe₂ in a crystallinephase when no information is recorded in the recording layer and afterinformation is erased therefrom.

From the aspect of the structure of the optical recording medium, it ispreferable that the above-mentioned optical recording medium comprise asubstrate in the form of a disk, and a first heat-resistant protectivelayer, the previously mentioned recording layer, a second heat-resistantprotective layer, and a reflective heat dissipation layer made of ametal or an alloy, which are overlaid on the substrate in this order.

Furthermore, in the above-mentioned optical recording medium, it ispreferable that the thickness of the first heat-resistant protectivelayer be in the range of 500 to 2500 Å; the thickness of the recordinglayer, 100 to 1000 Å; the thickness of the second heat-resistantprotective layer, 100 to 1500 Å, and the thickness of the reflectiveheat dissipation layer, 300 to 2000 Å.

Further, it is preferable that the disk-shaped substrate bear a guidegroove with a width of 0.25 to 0.65 μm and a depth of 250 to 650 Å.

In addition, the previously mentioned recording layer may furthercomprise a nitride and/or oxide comprising at least one of theconstituent elements Ag, In, Te and Sb, and in particular, a nitridecomprising Te with a bond of Te—N is preferable.

The fifth object of the present invention can be achieved by a method offorming a recording layer for an optical recording medium, comprisingthe step of sputtering a target which comprises a target materialcomprising as constituent elements Ag, In, Te and Sb with the respectiveatomic percents of α, β, γ and δ thereof being in the relationship of0.5≦α<8, 5≦β≦23, 17≦γ≦38, 32≦δ≦73, α≦β, and α+β+γ+δ=100, in anatmosphere of argon gas, with nitrogen gas being contained therein witha concentration of 0 to 10 mol %.

In the above-mentioned method of forming the recording layer, it ispreferable that the back pressure be set in the range of 3×10⁻⁷ to5×10⁻⁶ Torr prior to the sputtering step.

In addition, it is preferable that the partial pressure of the nitrogengas (P_(N)) be set in the range of 1×10⁻⁵ Torr≦(P_(N))≦8×10⁻⁵ Torr whenthe nitrogen gas is contained in the atmosphere during the sputteringstep.

Further, it is preferable that a mixed gas comprising argon gas andnitrogen gas with the concentration of the nitrogen gas being higherthan that in the atmosphere during the sputtering step be introducedinto a sputter chamber after the sputtering step.

The sixth object of the present invention can be achieved by an opticalrecording method for recording information in an optical recordingmedium comprising a disk-shaped substrate, and a first heat-resistantprotective layer, a recording layer comprising a phase-change recordingmaterial, a second heat-resistant protective layer, and a reflectiveheat dissipation layer comprising a metal or an alloy, which areoverlaid on the substrate in this order, the recording layer beingcapable of recording and erasing information by utilizing changes in thephase of the phase-change recording material in the recording layer, andthe phase-change recording material comprising as constituent elementsAg, In, Te and Sb with the respective atomic percents of α, β, γ and δthereof being in the relationship of 1≦α<6, 7≦β≦20, 20≦γ≦35, 35≦δ≦70,and α+β+γ+δ=100, comprising the step of applying a semiconductor laserbeam to the optical recording medium, with the optical recording mediumbeing rotated at a linear speed of 1.2 to 5.6 m/s.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic cross-sectional view showing the layer structureof a phase-change optical recording medium according to the presentinvention.

FIG. 2 is a graph which shows the overwriting characteristics of arecording layer of an optical recording medium according to the presentinvention, which recording layer is fabricated from a sputtering targetproduced by the method including the heat treatment step.

FIG. 3 is a graph which shows the overwriting characteristics of arecording layer of an optical recording medium according to the presentinvention, which recording layer is fabricated from a sputtering targetproduced by the method not including the heat treatment step.

FIG. 4 is a graph which shows the overwriting characteristics of anoptical recording medium according to the present invention, whoserecording layer is fabricated from a sputtering target comprisingAgInTe₂ crystallites with a particle size of 500 Å.

FIG. 5 is an FT-IR spectrum of an Ag—In—Sb—Te—N film which is obtainedby sputtering a target in an atmosphere of a mixed gas comprising argonand nitrogen with the concentration of the nitrogen being controlled to3 mol %, in comparison with that of an Ag—In—Sb—Te film which isobtained by sputtering the same target in an atmosphere of argon.

FIG. 6 is a graph which shows the relationship between the width anddepth of a guide groove formed on a substrate of the optical recordingmedium, and the push-pull magnitude after recording.

FIG. 7 is a graph which shows the relationship between the content ofnitrogen in a recording layer of an optical recording medium and therefractive index (n) of the recording layer.

FIG. 8 is a graph which shows the relationship between the content ofnitrogen in a recording layer of an optical recording medium and theabsorption coefficient (k) of the recording layer.

FIG. 9 is a graph which shows the relationship between the content ofnitrogen in a recording layer of an optical recording medium and thereflectance of the recording layer.

FIG. 10 is a graph which shows the relationship between the content ofnitrogen in a recording layer of an optical recording medium and thetransition speed from the crystalline phase to the amorphous phase ofthe recording layer.

FIG. 11 is a graph which shows the relationship between the baking timeof a substrate and the water content which can be removed from thesubstrate.

FIG. 12 is a graph which shows the relationship between the period oftime when a substrate is allowed to stand in a vacuum chamber and thedegree of vacuum in the vacuum chamber.

FIG. 13 is a graph which shows the relationship between the thickness ofthe recording layer and the jitter value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In order to obtain an optical recording medium of the present invention,a sputtering target for fabricating a recording layer of the opticalrecording medium can be employed. The sputtering target according to thepresent invention comprises a target material comprising as constituentelements Ag, In, Te and Sb, with the respective atomic percents (atom.%) of α, β, γ and δ thereof being in the relationship of 0.5≦α<8,5≦β≦23, 17≦γ≦38, 32≦δ≦73, α≦β, and α+β+γ+δ=100.

It is preferable that the above-mentioned target material comprise Sb,and AgInTe₂ with a stoichiometric composition and/or a nearlystoichiometric composition having a chalcopyrite structure and/orzincblende structure. In addition, it is preferable that theabove-mentioned AgInTe₂ crystallites have a particle size (d) of 450 Åor less.

When a recording layer for the optical recording medium is deposited ina thin film by sputtering by use of the aforementioned target, and thethus deposited recording layer is then subjected to proper heattreatment for initialization, the phase-change recording material withAg—In—Sb—Te system for use in the recording layer can assume a mixedphase, namely, a crystalline phase comprising AgSbTe₂ and an amorphousphase comprising In and Sb, as reported in “Proceedings of the 3rdSymposium on Phase-Change Recording” p. 102 (1991), and “JapaneseJournal of Applied Physics” vol. 32 (1993) pp. 5241-5247. When the mixedphase is formed in the phase-change recording material, it means that anamorphous phase comprising at least In and Sb is dispersed in a finecrystalline phase comprising AgSbTe₂ with a stoichiometric compositionand/or a nearly stoichiometric composition, and/or the crystallineAgSbTe₂ phase is dispersed in the amorphous phase of In and Sb. Anyway,since such a mixed phase of the phase-change recording material can beregarded as a non-recorded state, the thus obtained optical recordingmedium can be provided with high erasability, and is capable ofrepeatedly carrying out the recording operation and the erasingoperation by the application of a low power.

The above-mentioned mixed phase can be prepared in the recording layerbecause the target material of the sputtering target for fabricating therecording layer is a combination of AgInTe₂ and Sb. When the recordinglayer is deposited, the phase-change recording material for use in therecording layer is considered to initially comprise the AgInTe₂amorphous phase and the Sb amorphous phase. Thereafter, when therecording layer is initialized by the application of laser power orthermal energy thereto, the AgInTe₂ amorphous phase in the recordinglayer disappeared and changed into the AgSbTe₂ crystalline phase. Thus,there can be formed in the recording layer a mixed phase comprising thecrystalline phase of AgSbTe₂ with a stoichiometric and/or nearlystoichiometric composition and an amorphous phase comprising In and Sb.In other words, by the process of initializing, the Ag—In—Te—Sb basedrecording layer undergoes the substitution reaction, thereby inducing astructural change therein.

The particle size of the AgInTe₂ crystallites for use in the targetmaterial can be obtained by calculation, for instance, in accordancewith the Scherrer equation, from the width of a main peak which isobtained from the X-ray diffraction of the pulverized target. Forinstance, when Cu is employed for the source of X-ray and λ is about1.54 Å, the width of the main peak is about 24.1°. It is necessary thatthe width of the peak be corrected with reference to the main peak of areference sample with a sufficiently large particle size. In the casewhere the particle size of the AgInTe₂ crystallites in the sputteringtarget is 450 Å or less, it is possible to readily cause the recordinglayer to assume the previously mentioned mixed phase by subjecting therecording layer to appropriate heat treatment after deposition thereof,thereby performing stable recording and erasing operations.

To obtain the above-mentioned target for sputtering, a mixture of Ag, Inand Te elements is fused at a temperature in the range of 550° C. to850° C., rapidly cooled to prepare a solid lump, and pulverized toprepare finely-divided particles. The thus prepared finely-dividedparticles are mixed with Sb, and the mixture of the finely-dividedparticles and Sb is then sintered. Alternatively, a mixture of Ag, In,Te and Sb elements is fused at a temperature in the range of 550° C. to850° C., rapidly cooled and pulverized, and then the thus obtainedfinely-divided particles are sintered.

In any case, it is advantageous to subject the above-mentioned mixture,that is, the mixture of the finely-divided particles of Ag, In and Teelements and Sb, or the mixture of the finely-divided particles of Ag,In, Te and Sb elements, to heat treatment prior to the sintering step.The heat treatment may be carried out at a temperature not higher thanthe melting point of the mixture.

The optical recording medium according to the present inventioncomprises a recording layer comprising a phase-change recordingmaterial, capable of recording and erasing information by utilizingchanges in the phase of the phase-change recording material in therecording layer, the recording material comprising as constituentelements Ag, In, Te and Sb with the respective atomic percents of α, β,γ and δ thereof being in the relationship of 1≦α<6, 7≦β≦20, 20≦γ≦35,35≦δ≦70, and α+β+γ+δ=100.

Due to the phase-change recording material of a quaternary system ofAg—In—Te—Sb, the above-mentioned optical recording medium of the presentinvention exhibits high recording sensitivity and speed, that is,excellent phase change from a crystalline phase to an amorphous phase;high erasing sensitivity and speed, that is, excellent phase change froman amorphous phase to a crystalline phase; and high erasability. Toobtain the most satisfactory recording and erasing characteristics, itis preferable that the phase-change recording material for use in theoptical recording medium comprise AgSbTe₂ in a crystalline phase when noinformation is recorded in the recording layer and after information iserased therefrom.

In the present invention, the composition of the recording layer for usein the phase-change optical recording medium is measured based on theemission spectroscopic analysis. Not only the emission spectroscopicanalysis, but also X-ray microanalysis, Rutherford back scattering,Auger electron spectroscopic analysis, and X-ray fluorescence analysiscan be employed to measure the composition of the recording layer. Insuch a case, however, it is necessary to correct the obtainedcomposition with reference to the composition obtained by the emissionspectroscopic analysis. The composition of the recording layer specifiedin the present invention is the average one.

The X-ray diffraction or electron diffraction is appropriate for theobservation of the condition of the recording layer. Namely, when a spotpattern or Debye-Scherrer ring pattern is observed in the recordinglayer by the electron diffraction, the recording layer is considered toassume a crystalline state; while a ring pattern or halo pattern isobserved therein, the recording layer is considered to assume anamorphous state.

Further, the chemical bonding of the constituent elements in therecording layer, for example, the chemical bonding of an oxide ornitride existing in the recording layer can be efficiently analyzed bythe FT-IR or XPS.

From the aspect of structure of the optical recording medium, it ispreferable that the optical recording medium comprise a substrate in theform of a disk, and a first heat-resistant protective layer, thepreviously mentioned recording layer, a second heat-resistant protectivelayer and a reflective heat dissipation layer comprising a metal or analloy, which are successively overlaid on the substrate in this order.

The thickness of the recording layer deposited by use of the previouslymentioned sputtering target is preferably in the range of 100 to 1000 Å,more preferably in the range of 150 to 700 Å. When the thickness of therecording layer is in the range of 100 to 1000 Å, sufficient lightabsorbing capability of the recording layer can be maintained, and thephase change can uniformly and speedily take place in the recordinglayer.

Furthermore, when the jitter value is taken into consideration, the mostpreferable thickness of the recording layer is in the range of 170 to250 Å, as shown in FIG. 13. In FIG. 13, the composition of the recordinglayer is represented by the formula ofAg_(4.5)In_(14.5)Sb_(52.0)Te_(23.0)N_(6.0).

According to the present invention, the recording layer for the opticalrecording medium can be formed by sputtering the previously mentionedtarget of the present invention in an atmosphere of argon gas, withnitrogen gas being contained therein with a concentration of 0 to 10 mol%. According to the concentration of the nitrogen gas in the atmosphereduring the sputtering step, the composition of the obtained recordinglayer can be optimumly adjusted so as to be fitted to the desired diskrotation speed, the structure of layers of the recording disk, and theoperating conditions of the recording disk for practical use. By use ofa mixed gas of nitrogen gas and argon gas in the course of sputtering,the recording and erasing repetition reliability of the obtainedrecording medium is improved.

The mixed gas for use in the sputtering step may be prepared by mixingnitrogen gas and argon gas at a predetermined mixing ratio beforeintroduced into a sputter chamber. Alternatively, the argon gas and thenitrogen gas are introduced into the sputter chamber with the respectiveflow rates being controlled so as to obtain a desired molar ratio.

In this case, it is desirable that the partial pressure of the nitrogengas (P_(N)) be set in the range of 1×10⁻⁵ Torr≦(P_(N))≦8×10⁻⁵ Torr whenthe nitrogen gas is contained in the atmosphere during the sputteringstep.

It is also desirable that the back pressure (p) applied prior to thesputtering step be set in the range of 3×10⁻⁷≦(p)≦5×10⁻⁶ Torr. When theback pressure (p) is set so as to satisfy the above-mentionedrelationship of 3×10⁻⁷≦(p)≦5×10⁻⁶ Torr, the amorphous phase comprisingAgInTe₂ and Sb in the phase-change recording material can be readilychanged into the mixed phase of the crystalline phase of AgSbTe₂ and theIn—Sb amorphous phase in the recording layer by some heat treatmentafter deposition of the recording layer.

Further, it is advantageous that a mixed gas comprising argon gas andnitrogen gas with the concentration of the nitrogen gas being higherthan that in the atmosphere during the sputtering step be introducedinto the atmosphere after the sputtering step.

When the concentration of the nitrogen gas in the atmosphere is 10 mol %or less in the course of the sputtering step, the disk characteristicsof the obtained optical recording medium are excellent. To be morespecific, not only the increase of the maximum number of repeatedoverwriting operations, but also the modulation degree and the shelflife of a recorded mark (amorphous mark) can be improved. The mechanismof improvement in such disk characteristics caused by the addition of aproper amount of nitrogen to the recording layer has not been clarified,but it is supposed that when a proper amount of nitrogen is contained inthe recording layer, the density of the recording layer is decreased andvoids are increased, whereby the randomness of the recording layer isincreased in terms of the configuration. As a result, the degree oforder in the recording layer is moderated when compared with the casewhere no nitrogen is added to the recording layer. Therefore, thetransition from the amorphous phase to the crystalline phase tends to berestrained, so that the stability of the amorphous mark is increased andthe shelf life of the amorphous mark is improved. Furthermore, one ofthe advantages obtained by the addition of nitrogen to the recordinglayer is that the linear speed of the obtained optical recording mediumcan be controlled. To be more specific, the optimum linear speed of diskrotation can be lowered by the addition of nitrogen to the recordinglayer. Even though the composition of the target is identical, theoptimum linear speed of the phase-change optical recording disk can becontrolled simply by adjusting the mixing ratio of nitrogen gas to argongas introduced into the atmosphere during the sputtering step.

When the content of nitrogen in the recording layer is increased, therefractive index (n) of the recording layer is decreased and theabsorption coefficient (k) thereof is increased, as respectively shownin FIGS. 7 and 8. As a result, the reflectance of the obtained recordingmedium is greatly changed, as shown in FIG. 9. In addition, thetransition speed from the crystalline phase to the amorphous phase islowered as the content of nitrogen in the recording layer is increased,as shown in FIG. 10.

The recording layer of the optical recording medium according to thepresent invention may further comprise a nitride and/or oxide comprisingat least one of the constituent elements Ag, In, Te and Sb.

In particular, when the nitride comprises Te, for example, with a Te—Nor Sb—Te—N bond in the recording layer, the maximum number of repeatedoverwriting operations can be effectively increased. Such chemicalbonding in the recording layer can be analyzed by the FT-IR or XPS, aspreviously mentioned. For instance, a Te—N bond gives rise to peaks inthe 500 to 600 cm⁻¹ region; and absorptions arising from an Sb—Te—N bondoccur in the 600 to 650 cm⁻¹ region by the FT-IR analysis.

In the present invention, the recording layer for use in thephase-change optical recording medium may further comprise otherelements and impurities for the purpose of still more improvement of theperformance and increase of the reliability. For instance, elements asdisclosed in Japanese Patent Application 4-1488, that is, B, N, C, P andSi; and other elements such as O, S, Se, Al, Ti, V, Mn, Fe, Co, Ni, Cu,Zn, Ga, Sn, Pd, Pt and Au may be contained in the recording layer.

With respect to the influence of oxygen atom in the recording layer, themore the content of oxygen in the recording layer, the smaller therefractive index (n) of the recording layer and the larger theabsorption coefficient (k) thereof. In addition, the transition speedfrom the crystalline phase to the amorphous phase is lowered. However,unlike the nitrogen atom, the oxygen atom has an adverse effect on thelife of the recording medium and overwriting repetition properties whenthe content of the oxygen in the recording layer-is excessivelyincreased.

Particularly, when a substrate is made of a plastic material such aspolycarbonate resin, it is recommendable to carry out the followingsteps prior to the provision of the recording layer in order to controlthe content of oxygen atom in the recording layer to be provided on thesubstrate:

-   (1) Prior to the formation of a recording layer on the plastic    substrate, the substrate is subjected to baking at 60° C. or more    and atmospheric pressure to remove the water content from the    plastic substrate.

The moisture absorption of the substrate made of a plastic material suchas a polycarbonate resin is so high that the plastic substrate takes upwater even when the substrate is allowed to stand for several tens ofminutes after preparation by injection molding. If the substrate isplaced in the sputter chamber as it is in order to provide a recordinglayer thereon by sputtering process, oxygen is generated from the watercontent in the substrate, and incorporated in the recording layer to bedeposited. Therefore, it is preferable that the substrate be subjectedto baking at atmospheric pressure for dehydration before placed in thesputter chamber.

FIG. 11 is a graph which shows the relationship between the baking timeand the water content removed from a polycarbonate substrate. As can beseen from the graph of FIG. 11, it is effective to carry out the bakingof the substrate at 60° C. or more.

-   (2) Prior to the formation of a recording layer on the plastic    substrate, the substrate is allowed to stand in a vacuum chamber for    30 minutes or more to remove the water content from the plastic    substrate.

FIG. 12 is a graph which shows the relationship between the period oftime when the substrate is allowed to stand in the vacuum chamber andthe degree of vacuum in the vacuum chamber. As is apparent from thegraph, it is effective to allow the substrate to stand in the vacuumchamber for 30 minutes or more.

-   (3) Prior to the formation of a recording layer on the plastic    substrate, the substrate is subjected to etching by sputtering to    remove the water content from the plastic substrate, thereby    preventing the recording layer to be deposited thereon from being    contaminated by large amounts of oxygen atom.

It is preferable to carry out the etching of the plastic substrate in anatmosphere comprising at least argon gas or nitrogen gas at a pressureof 1×10⁻¹ to 8×10⁻¹ Pa.

The present invention will now be explained in detail with reference toFIG. 1.

FIG. 1 is a schematic cross-sectional view showing the layer structureof the phase-change optical recording medium according to the presentinvention. As shown in FIG. 1, a first heat-resistant protective layer2, a recording layer 3, a second heat-resistant protective layer 4 and areflective heat dissipation layer 5 are successively overlaid on asubstrate 1. It is not always necessary to provide both of the first andsecond heat-resistant protective layers 2 and 4. However, it isdesirable to provide the first protective layer 2 when a material withlow heat resistance, such as polycarbonate resin, is employed for thesubstrate 1.

In the present invention, glass, ceramic materials and resins are usableas the materials for the substrate 1 of the optical recording medium. Inparticular, the resin substrate is advantageous from the viewpoints ofmanufacturing cost and molding facility of a pregroove.

Examples of the resin as the material for the substrate 1 includepolycarbonate (PC) resin, polymethyl methacrylate (PMMA) resin,amorphous polyolefin (APO) resin, acrylic resin, epoxy resin,polystyrene resin, acrylonitrile—styrene copolymer resin, polyethyleneresin, polypropylene resin, silicone resin, fluoroplastics, ABS resinand urethane resin. Of those resins, the polycarbonate resin is the mostpractical material for the substrate 1 because of its goodprocessability and optical properties. The substrate may be prepared inthe form of a disk, card or sheet.

When the phase-change optical recording medium of the present inventionis applied to a rewritable compact disk (CD-Erasable), it is desirablethat the substrate 1 bear a guide groove with a width (half width) of0.25 to 0.65 μm, preferably 0.30 to 0.55 μm, and a depth of 250 to 650Å, preferably 300 to 550 Å.

When the optical recording medium of the present invention isconstructed with such layered structure as shown in FIG. 1, and thedisk-shaped substrate bears the above-mentioned guide groove, there canbe provided a rewritable compact disk with excellent compatibility.

With respect to the groove signal characteristics of the compact disk,the push-pull magnitude after recording (PPm) is prescribed by theOrange Book. According to the Orange Book, the push-pull magnitude afterrecording (PPm) of the compact disk is in the range of 0.04 to 0.15,preferably 0.04 to 0.12, and more preferably 0.04 to 0.09. It is verydifficult for the conventional phase-change rewritable compact disk tomeet both of the overall requirements for the recording and reproducingoperations, and the above-mentioned requirements of the groove signalcharacteristics. By using the phase-change optical recording medium ofthe present invention, however, a rewritable compact disk capable ofsatisfying both requirements can be provided.

The first and second heat-resistant protective layers 2 and 4 areprovided to protect the recording layer 3 from water and oxygen.Therefore, it is preferable that the materials for use in the first andsecond heat-resistant protective layers 2 and 4 have high corrosionresistance, low reactivity with the recording layer 3, and good adhesionto the substrate 1 and the recording layer 3.

Specific examples of the material for use in the first and secondheat-resistant protective layers 2 and 4 are metallic oxides such asSiO, SiO₂, ZnO, SnO₂, Al₂O₃, TiO₂, In₂O₃, MgO, and ZrO₂; nitrides suchas SiN, Si₃N₄, SiON, SiBN, SiBON, AlN, TiN, BN and ZrN; sulfides such asZnS, In₂S₃ and TaS₄; carbides such as SiC, TaC₄, B₄C, WC, TiC and ZrC;carbon with a diamond structure; and mixtures thereof, as disclosed inJapanese Patent Publication 4-74785. Further, those heat-resistantprotective layers may further comprise an impurity when necessary aslong as the melting points of the first and second protective layers 2and 4 are higher than that of the recording layer 3.

The first and second heat-resistant protective layers 2 and 4 for use inthe present invention can be provided by vacuum deposition, sputtering,plasma chemical vapor deposition, photochemical vapor deposition, ionplating or electron beam deposition method.

It is preferable that the thickness of the first heat-resistantprotective layer 2 be in the range of 500 to 2500 Å, more preferably inthe range of 1200 to 2300 Å. When the thickness of the firstheat-resistant protective layer 2 is within the range of 500 to 2500 Å,the first protective layer 2 can carry out its function, and thedecrease of sensitivity can be prevented, and the peeling of theprotective layer 2 does not easily take place. When necessary, the firstheat-resistant protective layer 2 may be of a multi-layered type.

It is preferable that the thickness of the second heat-resistantprotective layer 4 be in the range of 100 to 1500 Å, more preferably inthe range of 150 to 1000 Å. When the thickness of the secondheat-resistant protective layer 4 is within the range of 100 to 1500 Å,not only the second protective layer 4 can carry out its function, butalso the peeling of the second protective layer 4 can be prevented andthe decrease of the repetition recording properties can be preventedeven though the recording medium is rotated at a so-called low linearspeed, that is, in the range of 1.2 to 5.6 m/s. The secondheat-resistant protective layer 4 may also be of a multi-layered type.

The provision of the reflective heat dissipation layer 5 is not alwaysnecessary, but is desirable because the thermal energy excessivelyaccumulated during the recording and/or erasing operations can bedissipated by the reflective heat dissipation layer 5, thereby reducingthe thermal damage applied to the recording medium itself.

In view of the functions of the reflective heat dissipation layer 5, thematerials with high reflectance and a relatively high thermalconductivity may be employed.

Specific examples of the material for the reflective heat dissipationlayer 5 include metals such as Al, Au, Ag and Cu; and alloys thereof.

The reflective heat dissipation layer 5 can be provided by vacuumdeposition, sputtering, plasma chemical vapor deposition, photochemicalvapor deposition, ion-plating or electron beam deposition method.

It is preferable that the thickness of the reflective heat dissipation 5be in the range of 300 to 2000 Å, more preferably in the range of 500 to1500 Å.

Furthermore, the optical recording medium of the present invention mayfurther comprise an overcoat layer, for example, an ultraviolet-curingresin layer, which is provided on the reflective heat dissipation layer5. Those layers may be laminated with an adhesive.

As the electromagnetic waves for use in the present invention, which areused for initializing the obtained recording layer, and carrying out therecording, reproducing and erasing operations, there can be employedlaser beams, electron rays, X-rays, ultraviolet rays, visible rays,infrared rays, and microwave. In particular, the semiconductor laserbeam is most applicable as the electro-magnetic wave for use in therecording, reproducing and erasing operations because a system fordriving the optical recording medium can be made compact.

The optical recording operation can be satisfactorily performed byapplying a semiconductor laser beam to the previously mentioned opticalrecording medium, with the optical recording medium being rotated at alinear speed of 1.2 to 5.6 m/s.

Other features of this invention will become apparent in the course ofthe following description of exemplary embodiments, which are given forillustration of the invention and are not intended to be limitingthereof.

EXAMPLES 1 TO 8 AND COMPARATIVE EXAMPLES 1 TO 10

A 2000 Å thick first heat-resistant protective layer of ZnO.SiO₂ wasprovided on a polycarbonate substrate with a thickness of 1.2 mm bearingthereon a guide groove having a width of 0.5 μm, a depth of 600 Å and atrack pitch of 1.6 μm.

On the first heat-resistant protective layer, a recording layer with athickness of 250 Å was formed by sputtering by use of a target as shownin Table 1.

Further, a 300 Å thick second heat-resistant protective layer ofZnS.SiO₂, a 1000 Å thick reflective heat dissipation layer made of analuminum alloy comprising Si in an amount of 1.0 wt. %, and a 10 μmthick overcoat layer comprising an ultraviolet-curing resin weresuccessively overlaid on the recording layer.

Table 1 shows the composition of a sputtering target for fabricatingeach recording layer, which comprises Sb, and AgInTe₂ with astoichiometric composition and/or a nearly stoichiometric compositionhaving a chalcopyrite structure and/or zincblende structure; and thecomposition of the obtained recording layer.

The recording layer was formed on the first heat-resistant protectivelayer by sputtering under such conditions that the back pressure appliedprior to the sputtering process was set at 9×10⁻⁷ Torr, and thenadjusted to 4×10⁻³ Torr during the sputtering process by introducingargon gas into the sputter chamber, and the RF_(power) was set at 40 W.

Thus, disk-shaped optical recording media Nos. 1 to 8 according to thepresent invention and comparative disk-shaped optical recording mediaNos. 1 to 10 were fabricated.

The disk characteristics of each of the above-mentioned phase-changeoptical recording media Nos. 1 to 8 according to the present inventionand comparative phase-change optical recording media Nos. 1 to 10 wereevaluated by recording a mark and reproducing the recorded mark at suchan optimal disk rotation speed within a range of 1.2 to 5.6 m/s as toobtain a maximum C/N ratio. The wavelength of the semiconductor laserbeam was 780 nm, and Numerical Aperture (NA) value of the objective lenswas 0.5.

The signals were modulated in accordance with an Eight to FourteenModulation (EFM) system, and multi-pulse patterns were employed.

As a result of the above-mentioned evaluations, the disk characteristicsof each recording medium were assessed in accordance with the followingscale:

-   Level 3: Excellent disk characteristics.    C/N≧55 dB, and erasability (Ers)≦−35 dB.-   Level 2: Good disk characteristics.    55>C/N≧45 dB, and −35<(Ers)≦−25 dB.-   Level 1: Poor disk characteristics.    C/N<45 dB, and (Ers)>−25 dB.

The results of the disk characteristics are shown in Table 1.

In addition, the maximum number of repeated overwriting operations wasmeasured to evaluate the overwriting characteristics. The results arealso shown in Table 1.

TABLE 1 Maximum Composition of Sputtering Level of Number of TargetComposition of Recording Layer Disk Repeated Ag In Te Sb Ag In Te SbCharacter- Overwriting (at. %) (at. %) (at. %) (at. %) (at. %) (at. %)(at. %) (at. %) istics Operations Ex. 1 4.0 15.0 25.0 56.0 2.0 13.0 24.061.0 3 12,000 Ex. 2 4.0 11.0 28.0 57.0 4.0 12.0 27.0 57.0 3 15,000 Ex. 37.0 11.0 22.0 60.0 5.5 9.0 21.0 64.5 3 7,000 Ex. 4 1.5 10.0 28.0 60.52.0 11.0 28.0 59.0 3 8,000 Ex. 5 3.0 8.0 19.0 70.0 1.0 7.0 22.0 70.0 310,000 Ex. 6 0.7 5.0 30.0 64.3 1.0 7.0 28.0 64.0 2 8,000 Ex. 7 4.0 8.023.0 65.0 2.0 7.0 21.0 70.0 2 10,000 Ex. 8 6.0 21.0 20.0 53.0 4.0 19.020.0 57.0 2 13,000 Comp. 1.0 10.0 15.0 74.0 1.0 10.0 13.0 76.0 1 ≦200Ex. 1 Comp. 2.0 3.0 10.0 85.0 1.5 4.0 8.0 86.5 1 ≦200 Ex. 2 Comp. 8.020.0 55.0 17.0 11.0 17.0 63.0 9.0 1 ≦200 Ex. 3 Comp. 10.0 1.0 40.0 49.07.0 0.1 37.0 55.9 1 ≦200 Ex. 4 Comp. 11.0 9.0 30.0 50.0 8.0 7.0 26.059.0 1 ≦200 Ex. 5 Comp. 15.0 6.0 5.0 74.0 13.0 5.0 4.0 78.0 1 ≦200 Ex. 6Comp. 27.0 23.0 40.0 10.0 20.0 31.0 45.0 4.0 1 ≦200 Ex. 7 Comp. 27.0 8.025.0 40.0 32.0 5.0 25.0 38.0 1 ≦200 Ex. 8 Comp. 10.0 15.0 28.0 47.0 7.013.0 26.0 54.0 3 ≦500 Ex. 9 Comp. 11.0 11.0 22.0 56.0 8.0 9.0 21.0 62.03 ≦500 Ex. 10

As can be seen from the results shown in Table 1, the diskcharacteristics of the optical recording medium are satisfactory whenthe sputtering target for fabricating the recording layer comprises atarget material comprising as constituent elements Ag, In, Te and Sb,with the respective atomic percents of α, β, γ and δ thereof being inthe relationship of 0.5≦α<8, 5≦β≦23, 17≦γ≦38, 32≦δ≦73, α≦β, andα+β+γ+δ=100, and the composition of the obtained recording layersatisfies the relationship of 1≦α<6, 7≦β≦20, 20≦γ≦35, 35≦δ≦70, andα+β+γ+δ=100.

Each sputtering target for fabricating the recording layer employed inExamples 1 to 8 was produced by fusing the constituent elements of Ag,In, Te and Sb to prepare a fused mixture, rapidly cooling the fusedmixture to prepare a solid lump, pulverizing the solid lump to preparefinely-divided particles, subjecting the particles to heat treatment,and sintering the finely-divided particles.

FIG. 2 is a graph which shows the overwriting characteristics of theoptical recording medium No. 5 fabricated in Example 5 when theoverwriting operation was carried out at a linear speed of 5 m/sec.

EXAMPLE 9

The procedure for fabricating the disk-shaped optical recording mediumNo. 5 in Example 5 was repeated except that the method of producing thetarget for fabricating the recording layer of the recording medium No. 5in Example 5 did not include the heat treatment step prior to thesintering step. Thus, a disk-shaped optical recording medium No. 9according to the present invention was fabricated.

The overwriting characteristics of the optical recording medium No. 9were evaluated at a linear speed of 5 m/sec in the same manner as inExample 5.

FIG. 3 is a graph which shows the overwriting characteristics of theoptical recording medium No. 9.

In comparison with the graph of FIG. 3, the optical recording medium No.5 is superior to the recording medium No. 9 with respect to the diskcharacteristics, such as the C/N ratio and the erasability, and theoverwriting sensitivity.

EXAMPLES 10 TO 12

The procedure for fabricating the disk-shaped optical recording mediumNo. 3 in Example 3 was repeated except that nitrogen gas was containedin the atmosphere with concentrations of 3.0, 6.0, and 10.0 mol %,respectively in Example 10, Example 11, and Example 12 when therecording layer was formed on the first heat-resistant protective layerby sputtering.

Thus, disk-shaped optical recording media Nos. 10 to 12 according to thepresent invention were fabricated.

Table 2 shows the composition of each of the thus obtained recordinglayers in terms of the atomic percents of constituent elements.

In the same manner as in Example 1, each of the optical recording mediaNos. 3, 10, 11 and 12 according to the present invention was repeatedlysubjected to the overwriting operation, and the maximum number of therepeated overwriting operations was obtained.

The results are also shown in Table 2.

TABLE 2 Maximum Number of Composition of Optimal Repeated N₂/ RecordingLayer Recording Overwriting Example (Ar + N₂) (atom. %) SensitivityOperations No. (mol %) Ag In Te Sb (mW) *1) *2) Ex. 3  0 5.5 9.0 21.064.5 12 7,000 Ex. 10 3 5.2 8.5 20.0 66.3 12 15,000 Ex. 11 6.0 5.3 8.822.0 63.9 13 20,000 Ex. 12 10.0 5.5 9.2 21.2 64.1 14 8,000 *1) Theoptimal recording sensitivity is expressed by a laser power forrecording with which the jitter value is minimized. It is desirable thatthe optimal recording sensitivity be 15 mW or less. *2) The maximumnumber of repeated overwriting operations is expressed by the number ofoverwriting cycles just before there is a sudden increase in jittervalue. It is desirable that the maximum number of repeated overwritingoperations be 1,000 times or more.

As is apparent from the results shown in Table 2, the maximum number ofrepeated overwriting operations of the optical recording medium issatisfactory when the concentration of the nitrogen gas is 10 mol % orless in the sputter chamber in the course of the formation of therecording layer by sputtering.

In addition, an oxide comprising In, or a nitride comprising In or Tewas observed in the obtained recording layer when the nitrogen gas wasadded to the argon gas in the sputtering process. In particular, it wasconfirmed that the chemical bonding of Te—N or Sb—Te—N in the recordinglayer has considerable effect on the increase of the maximum number ofrepeated overwriting operations.

A sample recording layer with a thickness of 1000 Å was provided on anSi-wafer by sputtering using the sputtering target in a sputter chamber,with the concentration of nitrogen gas in the mixed gas (Ar+N₂) beingcontrolled to 3 mol %. The sample of the recording layer thus obtainedwas analyzed by the FT-IR. FIG. 5 shows the FT-IR spectrum of the aboveobtained sample in comparison with that of a sample of the recordinglayer which was deposited on the Si-wafer by sputtering using the sametarget with no nitrogen being contained in the atmosphere during thesputtering process.

In the above-mentioned FT-IR analysis, a commercially availableinstrument “IFS-113 V” (made by Bruker Corp.) was employed.

EXAMPLE 13

It was confirmed that each of the sputtering targets employed inExamples 1 to 8 for fabricating each recording layer comprises AgInTe₂crystallites with a particle size of 450 Å or less.

The procedure for fabricating the disk-shaped optical recording mediumNo. 2 in Example 2 was repeated except that the target for fabricatingthe recording layer of the recording medium No. 2 in Example 2 wasreplaced by a target with the same composition as that of the targetemployed in Example 2, with the particle size of the AgInTe₂crystallites for use in the target material being controlled to 500 Å.

Thus, a disk-shaped optical recording medium No. 13 according to thepresent invention was fabricated.

The overwriting characteristics of the optical recording medium No. 13were compared with those of the optical recording medium No. 2 bycarrying out the overwriting operation at a disk rotation speed of 2m/s.

FIG. 4 is a graph which shows the overwriting characteristics of theoptical recording medium No. 13.

As a result, the optical recording medium No. 2 is superior to therecording medium No. 13 with respect to

As a result, the optical recording medium No. 2 is superior to therecording medium No. 13 with respect to the disk characteristics such asthe C/N ratio and the erasability.

COMPARATIVE EXAMPLES 11 AND 12

In the preparation of the sputtering target for fabricating eachrecording layer in Examples 1 to 8, a mixture of the constituentelements Ag, In, Te and Sb was fused at a temperature in the range of550° C. to 850° C.

The procedure for preparation of the sputtering target as employed inExample 2 was repeated except that a mixture of the constituent elementsAg, In, Te and Sb was fused at 500° C. and 900° C., respectively inComparative Examples 11 and 12, so that comparative sputtering targetswere fabricated.

The recording layer was formed using each of the above-mentioned twokinds of sputtering targets, and then comparative disk-shaped opticalrecording media Nos. 11 and 12 were fabricated in the same manner as inExample 2.

The recording and erasing characteristics of the comparative opticalrecording medium No. 11 were unsatisfactory for the practical use, andthose of the comparative optical recording medium No. 12 were inferiorto those of the optical recording medium No. 2.

EXAMPLE 14

The procedure for fabricating the disk-shaped optical recording mediumNo. 2 in Example 2 was repeated except that the width and depth of theguide groove provided on the polycarbonate substrate were variouslychanged as shown in FIG. 6.

Thus, disk-shaped optical recording media according to the presentinvention were fabricated.

As can be seen from the graph shown in FIG. 6, satisfactory groovesignal characteristics are obtained when the width of the guide grooveis in the range of 0.25 to 0.65 μm and the depth of the guide groove isin the range of 250 to 650 Å.

EXAMPLE 15

The procedure for fabricating the disk-shaped optical recording mediumNo. 2 in Example 2 was repeated except that the thicknesses of thelayers constituting the optical recording medium No. 2 were variouslychanged as shown in Table 3.

Thus, disk-shaped optical recording media No. 15-1 to No. 15-27according to the present invention were fabricated.

Each of the optical recording media Nos. 15-1 to 15-27 according to thepresent invention was repeatedly subjected to the overwriting operation,and the maximum obtained.

In addition, the optimal recording sensitivity of each recording mediumwas obtained in the same manner as in Example 10.

The results are also shown in Table 3.

TABLE 3 Thickness (Å) First Second Reflective heat- Re- heat- heatOptimal Maximum Number resistant cord- resistant dissi- Recording ofRepeated Example protective ing protective pation SensitivityOverwriting No. layer layer layer layer *1) (mW) Operations *2) Ex. 22,000 250 300 1,000 12 15,000 Ex. 15-1 1,500 250 300 1,000 12 15,000 Ex.15-2 1,800 200 250 1,000 13 20,000 Ex. 15-3 2,400 200 250 1,000 14 8,000 Ex. 15-4 500 200 250 1,000 14  1,000 Ex. 15-5 400 200 250 1,00016   300 Ex. 15-6 2,700 200 250 1,000 15   800 Ex. 15-7 2,000 80 2501,000   16<   600 Ex. 15-8 2,000 110 250 1,000 15  1,000 Ex. 15-9 2,000300 250 1,000 13 12,000 Ex. 15-10 2,000 400 250 1,000 13 10,000 Ex.15-11 2,000 700 250 1,000 12  8,000 Ex. 15-12 2,000 1,000 250 1,000 11 5,000 Ex. 15-13 2,000 1,200 250 1,000 10   700 Ex. 15-14 2,000 200 801,000 16   400 Ex. 15-15 2,000 200 120 1,000 15  1,500 Ex. 15-16 2,000200 400 1,000 13  7,000 Ex. 15-17 2,000 200 600 1,000 13  4,000 Ex.15-18 2,000 200 800 1,000 12  3,000 Ex. 15-19 2,000 200 1,000 1,000 11 2,000 Ex. 15-20 2,000 200 1,300 1,000 10  1,500 Ex. 15-21 2,000 2001,600 1,000 10   600 Ex. 15-22 2,000 200 250 2,400   16<  1,000 Ex.15-23 2,000 200 250 1,900 15  1,500 Ex. 15-24 2,000 200 250 1,300 13 7,000 Ex. 15-25 2,000 200 250 700 12  9,000 Ex. 15-26 2,000 200 250 40011  1,200 Ex. 15-27 2,000 200 250 250 11     200>

As can be seen from the results shown in Table 3, the recordingsensitivity becomes high and the maximum number of repeated overwritingoperations can be increased when the thickness of the firstheat-resistant protective layer is in the range from 500 to 2500 Å; thatof the recording layer, from 100 to 1000 Å; that of the secondheat-resistant protective layer, from 100 to 1500 Å; and that of thereflective heat dissipation layer, from 300 to 2000 Å.

To be more specific with reference to Table 3, the effect of the optimalthickness of the first heat-resistant protective layer is shown in theresults of Examples 15-1 to 15-6; the effect of the optimal thickness ofthe recording layer, in Examples 15-7 to 15-13; the effect of theoptimal thickness of the second heat-resistant protective layer, inExamples 15-14 to 15-21; and the effect of the optimal thickness of thereflective heat dissipation layer, in Examples 15-22 to 15-27.

As previously explained, the phase-change optical recording medium ofthe present invention achieves high performance for C/N ratio,erasability, recording and erasing sensitivities, and repetitionreliability.

The advantages of the above-mentioned properties can be obtained fromthe following factors:

-   (1) The recording layer comprises the phase-change recording    material comprising as constituent elements Ag, In, Te and Sb with    the respective atomic percents of α, β, γ and δ thereof being in the    relationship of 1≦α<6, 7≦β≦20, 20≦γ≦35, 35≦δ≦70, and α+β+γ+δ=100.    Therefore, there can be obtained a recording layer with excellent    overall characteristics required for the phase-change optical    recording disk at a low linear speed of 1.2 to 5.6 m/s.-   (2) When a first heat-resistant protective layer, the    above-mentioned recording layer, a second heat-resistant protective    layer and a reflective heat dissipation layer are overlaid on a    substrate in this order, the phase-change optical recording medium    with excellent overall characteristics can be obtained.-   (3) When the substrate bears thereon a guide groove with a width of    0.25 to 0.65 μm and a depth of 250 to 650 Å, the compatibility of    the obtained optical recording disk with the CD-ROM or the CD-R can    be easily ensured.-   (4) In the case where the phase-change recording material for use in    the recording layer comprises AgSbTe₂ in a crystalline phase when no    information is recorded in the recording layer and after information    is erased therefrom, the erasing characteristics of the recording    medium can be improved.-   (5) When the first heat-resistant protective layer has a thickness    of 500 to 2500 Å, the recording layer has a thickness of 100 to 1000    Å, the second heat-resistant protective layer has a thickness of 100    to 1500 Å, and the reflective heat dissipation layer has a thickness    of 300 to 2000 Å, the overall characteristic of the obtained    phase-change optical recording medium are excellent at a low linear    speed of 1.2 to 5.6 m/s.-   (6) By applying the phase-change optical recording medium of the    present invention to an optical recording method, the method capable    of recording information with a large capacity can be provided.-   (7) By use of the phase-change optical recording medium of the    present invention, there can be provided an optical recording method    suitable for CD-rewritable application or the modified application    thereof.-   (8) A sputtering target for fabricating a recording layer of an    optical recording medium comprises a target material comprising as    constituent elements Ag, In, Te and Sb with the respective atomic    percents (atom. %) of α, β, γ and δ thereof being in the    relationship of 0.5≦α<8, 5≦β≦23, 17≦γ≦38, 32≦δ≦73, α≦β, and    α+β+γ+δ=100. Therefore, excellent phase-change optical recording    medium can be provided by using such a target as a starting material    for the recording layer.-   (9) When the target material for use in the above-mentioned    sputtering target comprises Sb, and AgInTe₂ with a stoichiometric    composition and/or a nearly stoichiometric composition having a    chalcopyrite structure and/or zincblende structure, excellent    phase-change optical recording medium can be provided.-   (10) Further, when the crystallites of AgInTe₂ in the target    material have a particle size of 450 Å or less, excellent    phase-change optical recording medium can be provided.-   (11) The above-mentioned sputtering target can be produced by fusing    a mixture of Ag, In and Te elements at a temperature in the range of    550° C. to 850° C. to prepare a fused mixture; rapidly cooling the    fused mixture to prepare a solid lump; pulverizing the solid lump to    prepare finely-divided particles; mixing the finely-divided    particles with Sb; and sintering the mixture of the finely-divided    particles and Sb.-   (12) Alternatively, the sputtering target can also be produced by    fusing-a mixture of Ag, In, Te and Sb elements at a temperature in    the range of 550° C. to 850° C. to prepare a fused mixture; rapidly    cooling the fused mixture to prepare a solid lump; pulverizing the    solid lump to prepare finely-divided particles; and sintering the    finely-divided particles.-   (13) When the step of subjecting the mixture of the finely-divided    particles to heat treatment at a temperature not higher than the    melting point of the mixture is included prior to the sintering step    in the above-mentioned method of producing the sputtering target,    the obtained phase-change optical recording medium is stable.-   (14) According to the present invention, a recording layer for an    optical recording medium can be formed by sputtering the previously    mentioned target in an atmosphere of argon gas, with nitrogen gas    being contained therein with a concentration of 0 to 10 mol %.-   (15) A small amount of nitrogen gas to be contained in the recording    layer can be controlled with no difficulty when the partial pressure    of the nitrogen gas (P_(N)) is set in the range of 1×10⁻⁵    Torr≦(P_(N))≦8×10⁻⁵ Torr during the sputtering step.-   (16) When the back pressure p is set in the range of 3×10⁻⁷≦p≦5×10⁻⁶    Torr prior to the sputtering step in the above-mentioned method of    forming the recording layer for the optical recording medium, there    can be obtained a phase-change optical recording medium with high    performance and improved stability.-   (17) When there is introduced into the atmosphere after the    sputtering step a mixed gas comprising argon gas and nitrogen gas    with the concentration of the nitrogen gas being higher than that    during the sputtering step, there can be obtained a phase-change    optical recording medium with high performance and improved    stability.-   (18) When the recording layer of the phase-change optical recording    medium according to the present invention further comprises a    nitride and/or oxide comprising at least one of the constituent    elements Ag, In, Te and Sb, the characteristics of the obtained    recording medium are further improved, in particular, with respect    to the maximum number of repeated overwriting operations and the    repetition reliability.-   (19) When the above-mentioned nitride in the recording layer    comprises Te with a Te—N bond, the maximum number of repeated    overwriting operations can be further increased and the optimal    linear speed of disk rotation can be easily controlled as desired.

1. A method of producing a sputtering target which comprises a targetmaterial comprising as constituent elements Ag, In, Te and Sb with therespective atomic percents (atom. %) of α, β, γ and δ thereof being inthe relationship of:0.5≦α<8,5≦β≦23,17≦γ≦38,32≦δ≦73,α≦β, andα+β+γ+δ=100. comprising the steps of: fusing a mixture of Ag, In and Teelements to prepare a fused mixture; cooling said fused mixture toprepare a solid lump; pulverizing said solid lump to preparefinely-divided particles; mixing said finely-divided particles with Sb;and sintering the mixture of said finely-divided particles and Sb.